Image sensor

ABSTRACT

An image sensor includes a control circuit and pixels. Each pixel includes: a photosensitive area, a substantially rectangular storage area adjacent to the photosensitive area, and a read area. First and second insulated vertical electrodes electrically connected to each other are positioned opposite each other and delimit the storage area. The first electrode extends between the storage area and the photosensitive area. The second electrode includes a bent extension opposite a first end of the first electrode, with the storage area emerging onto the photosensitive area on the side of the first end. The control circuit operates to apply a first voltage to the first and second electrodes to perform a charge transfer, and a second voltage to block charge transfer.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. application for patent Ser.No. 15/488,691 filed Apr. 17, 2017, which is a continuation of U.S.application for patent Ser. No. 15/136,569 filed Apr. 22, 2016 (now U.S.Pat. No. 9,673,247), which claims the priority benefit of FrenchApplication for Patent No. 1560422, filed on Oct. 30, 2015, thedisclosures of which are hereby incorporated by reference.

TECHNICAL FIELD

The present disclosure relates to an image sensor comprising a pluralityof pixels formed inside and on top of a semiconductor substrate. Asensor adapted to a so-called global shutter control mode is here morespecifically considered.

BACKGROUND

In an image sensor adapted to a global shutter control, each pixelcomprises a photosensitive area, a read area, and a storage area. Thephotogenerated charges accumulated during an integration phase in thephotosensitive areas of all the sensor pixels are simultaneouslytransferred into the corresponding storage areas and a full image isthen stored in all the sensor storage areas. The stored image may thenbe read, line by line, during the next integration phase. Pixelstructures compatible with a global shutter control where insulatedvertical electrodes enable to transfer charges from the photosensitivearea to the storage area, and from the storage area to the read area,are taught by U.S. Pat. No. 9,236,407 (incorporated by reference).

FIGS. 1A to 1G are copies of FIGS. 3A to 3G of U.S. Pat. No. 9,236,407illustrating an example of a pixel 200 formed inside and on top of asilicon semiconductor substrate 201. FIG. 1A shows this pixel in topview. FIGS. 1B to 1G are cross-section views respectively along planesB-B, C-C, D-D, E-E, F-F, and G-G of FIG. 1A.

Pixel 200 comprises a photosensitive area, an intermediate chargestorage area, and a read area connected to a read and processingcircuit.

Substrate 201 is lightly P-type doped (P−). The photosensitive area ofpixel 200 comprises an N-type doped well 205, having a doping level N1,forming with substrate 201 the junction of a photodiode, or photosite,PD′. The storage area of the pixel comprises, juxtaposed to well 205, anN-type doped well 207, having a doping level N2 greater than N1, formingwith substrate 201 the junction of a diode SD′. Wells 205 and 207substantially have the same depth and have a common side. A thinheavily-doped P-type layer 213 (P+) is formed at the surface of wells205 and 207 so that photosite PD′ and diode SD′ are of pinned type. Theread area of pixel 200 comprises, adjacent to well 207, on the side ofwell 207 opposite to well 205, a region 211 formed at the surface ofsubstrate 201 and more heavily N-type doped (N⁺) than wells 205 and 207.

An insulated vertical electrode 203 extends in the substrate down to adepth greater than that of wells 205 and 207, between wells 205 and 207,at the level of their common side. Electrode 203 insulates well 205 fromwell 207, except in a charge transfer area 204 where electrode 203comprises an opening extending along its entire height and connectingwell 205 to well 207. Electrode 203 has, in top view, a U shapedelimiting most of three sides of well 207, the horizontal line of the Ubeing located at the level of the side common to well 205 and 207.

Another insulated vertical electrode 209 extends in the substratebetween well 207 and reading region 211, at the level of their commonside, down to a depth greater than that of well 207. Electrode 209insulates well 207 from region 211, except at the level of a chargetransfer area 206 where electrode 209 comprises an opening extendingalong its entire height and connecting well 207 to region 211. Electrode209 has the shape of a vertical plane delimiting most of the side ofwell 207 adjacent to region 211 (that is, the side of well 207 oppositeto transfer area 204).

Another insulated vertical electrode 202 extending down to a depth atleast equal to that of well 205 laterally delimits most of the threesides of well 205 which are not delimited by electrode 203.

Electrodes 202, 203, and 209 and region 211 are connected bymetallizations (not shown), respectively to a node V_(P), to nodes TG1and TG2, and to a node SN connected or coupled to a read circuit. Readcircuit (FIG. 1A) comprises a transistor 213 connecting node SN to ahigh power supply rail V_(DD) of the sensor, a transistor 215 assembledas a source follower, having its gate connected to node SN, and havingits drain connected to rail V_(DD), and a transistor 217 connecting thesource of transistor 215 to a reading line 219 of an array networkcomprising pixel 200. The gate of transistor 213 is connected to a nodeRST of application of a signal for resetting region 211, and the gate oftransistor 217 is connected to a node RS of application of a pixelselection signal 200. Transistors 213, 215, and 217 are formed in aP-type doped well 220 (PW), laterally delimited by an insulating region221.

In charge accumulation or integration phase, nodes V_(P) and TG1 are ata same low voltage in the order of −1 V. Such a biasing of electrodes202 and 203 causes an accumulation of holes along the walls of thevertical trenches delimiting the photosensitive area. Holes alsoaccumulate in transfer area 204, thus blocking electron exchangesbetween wells 205 and 207. Since substrate 201 is biased to the groundvoltage, a potential well forms in well 205, which, in the absence ofillumination, depends on the doping levels and on the bias voltages ofthe electrodes and of the substrate. When photodiode PD′ is illuminated,electron/hole pairs are photogenerated in the photodiode, and thephotogenerated electrons are attracted towards well 205 and trappedtherein.

In a phase of transfer of the photogenerated electrons accumulated inwell 205 to the storage area, node TG1 is set to a high voltage suchthat the depletion voltage of transfer area 204 has a value greater thanthe maximum potential of the potential well in photodiode PD′ totransfer the electrons contained in well 205 into well 207, via transferarea 204. Node VP is maintained at the low voltage. Node TG2 is also ata low voltage, which causes the accumulation of holes in transfer area206, thus blocking electron exchanges between well 207 and region 211.Once the transfer has been performed, node TG1 is set back to the lowvoltage, to maintain the transferred electrons confined in well 207. Atthis stage, a new integration phase may start.

In read phase, the charges contained in well 207 are transferred to readarea 211, via transfer area 206. To achieve this, node TG2 is set to ahigh voltage such that the depletion voltage in transfer area 206 has avalue greater than the maximum potential of the potential well in diodeSD′. Nodes V_(P) and TG1 are maintained at the low voltage. Once thetransfer has been performed, node TG2 is set back to the low voltage.

A pixel of type in FIGS. 1A to 1G suffers from various disadvantages,especially as concerns the charge transfer from the photosensitive areato the storage area.

It would thus be desirable to have a pixel structure compatible with aglobal shutter control which overcomes at least some of thedisadvantages of existing structures.

SUMMARY

Thus, an embodiment provides an image sensor arranged inside and on topof a semiconductor substrate, comprising a control circuit and aplurality of pixels, each pixel comprising: a photosensitive area, anelongated storage area at least five times longer than it is wide andadjacent to the photosensitive area, and a read area separated from thestorage area by a portion of the substrate; a first and a secondinsulated vertical electrodes electrically connected to each other,extending in the substrate opposite each other, and laterally delimitingthe storage area, the first electrode extending between the storage areaand the photosensitive area, the second electrode comprising a bentextension opposite a first end of the first electrode, the storage areaemerging onto the photosensitive area on the side of the first end andon said portion of the substrate on the side of the second end of thefirst electrode, the control circuit being capable of applying a firstvoltage to the first and second electrodes to perform a charge transferfrom the photosensitive area to the storage area, and a second voltagefor blocking said transfer.

According to an embodiment, each pixel further comprises a thirdinsulated vertical electrode extending in the substrate opposite thefirst electrode, short of the first end and beyond the second end, andpartially delimiting the photosensitive area on the side of the storagearea.

According to an embodiment, the control circuit is capable of applyingthe second voltage to the third electrode.

According to an embodiment, each pixel further comprises at least onefourth insulated vertical electrode extending in the substrate andpartially surrounding the photo-sensitive area.

According to an embodiment, the control circuit is capable of applyingthe second voltage to said at least one fourth electrode.

According to an embodiment, each pixel further comprises an insulatedcontrol gate arranged on top of and in contact with said portion of thesubstrate, the insulated gate extending from the read area to thestorage area.

According to an embodiment, the control circuit is capable of applying athird voltage to the control gate to transfer charges from the storagearea to the read area.

According to an embodiment, the substrate is doped with a firstconductivity type, the read area is doped with the second conductivitytype, the photosensitive area comprises a first doped well of the secondconductivity type coated with a doped layer of the first conductivitytype, and the storage area comprises a second doped well of the secondconductivity type and at least partially coated with said doped layer ofthe first conductivity type.

According to an embodiment, the first well extends all the way to thesecond well.

According to an embodiment, the thickness of the first well is smallerthan 1 μm.

According to an embodiment, the width of the storage area is in therange from 0.1 to 1 μm.

According to an embodiment, the storage area is trapezoidal and is wideron the side of the second end of the first electrode than on the side ofthe first end of the first electrode.

According to an embodiment, the storage area is rectangular.

According to an embodiment, the storage area comprises at least twoelongated regions at least five times longer than they are wideseparated from one another by at least one fifth insulated verticalelectrode extending in the substrate between the first and secondelectrodes and having ends aligned with the ends of the first electrode.

According to an embodiment, the width of each of said at least twoelongated regions of the storage area is in the range from 0.1 to 1 μm.

BRIEF DESCRIPTION OF THE DRAWINGS

The foregoing and other features and advantages will be discussed indetail in the following non-limiting description of specific embodimentsin connection with the accompanying drawings, wherein:

FIGS. 1A to 1G, previously described, schematically illustrate anexample of a pixel compatible with a global shutter control;

FIGS. 2A to 2E schematically illustrate an embodiment of a pixelcompatible with a global shutter control;

FIGS. 3A and 3B illustrate the shape of the voltage in a storage arearespectively of the pixel of FIGS. 1A to 1G and of the pixel of FIGS. 2Ato 2E;

FIGS. 4A and 4B illustrate the shape of the voltage in a photosensitivearea respectively of the pixel of FIGS. 1A to 1G and of the pixel ofFIGS. 2A to 2E; and

FIGS. 5A and 5B schematically illustrate an alternative embodiment ofthe pixel of FIGS. 2A to 2E.

DETAILED DESCRIPTION

The same elements have been designated with the same reference numeralsin the different drawings and, further, the various drawings are not toscale.

In the following description, terms “front”, “back”, “left”, “right”,“top”, “upper”, “lower”, “horizontal”, “vertical” refer to theorientation of the concerned elements in the corresponding drawings.Unless otherwise specified, expressions “approximately”,“substantially”, and “in the order of” mean to within 10%, preferably towithin 5%.

FIGS. 2A to 2E illustrate an embodiment of a pixel 300. FIG. 2A showsthis pixel in top view. FIGS. 2B to 2E are cross-section viewsrespectively along planes B-B, C-C, D-D, and E-E of FIG. 2A.

Pixel 300 is formed inside and on top of a semiconductor substrate 301,for example, made of silicon, the substrate being lightly P-type doped(P⁻) in this example. Pixel 300 comprises a photosensitive area, acharge storage area, and a read area. The pixel further comprises a readcircuit (not shown) having the read area coupled or connected thereto.

The photosensitive area of pixel 300 comprises an N-type doped well orlayer 303, of doping level N3, forming with substrate 301 the junctionof a photodiode, or photosite, PD. Well 303 is coated, at the level ofthe upper surface of the substrate, with a heavily-doped P-type thinlayer 305 (P⁺). Thus, photodiode PD is a vertically pinned photodiode.Well 303 is thinner than well 205 of pixel 200. In this example, well303 is approximately parallelepipedal.

The storage area of pixel 300 comprises an N-type doped well 307, ofdoping level N4, forming with substrate 301 the junction of a diode SD.In this example, well 307 penetrates deeper into substrate 301 than well303. At the upper surface level of the substrate, well 307 is coatedwith P⁺-type thin layer 305. Thus, diode SD is a vertically pinneddiode. In top view, well 307 has the shape of a rectangle having alength at least five times greater than its width. The dimensions anddoping levels N3 and N4 of wells 303 and 307 are selected so that thestorage capacity of diode SD is greater than or equal to that ofphotodiode PD. The storage area is adjacent to the photosensitive area,more particularly to an edge of the photosensitive area, and extendslengthwise in a direction parallel to this edge. Insulated verticalelectrodes 309 and 311 laterally delimit the storage area, that is, thestorage area extends widthwise from electrode 309 to electrode 311.Electrodes 309 and 311 extend in the substrate down to a same depth,preferably down to a depth greater than or equal to that of well 307.Electrode 309 has the shape of a vertical section arranged between thephotosensitive area and the storage area and fully delimits a first longside of the storage area. Electrode 311 comprises a portion 311Aparallel to electrode 309 and at least partially opposite the latter sothat portion 311A fully delimits, on the side opposite to electrode 309,a second long side of the storage area. Electrode 311 further comprisesa portion or extension 311B extending all the way to the photosensitivearea to be opposite a first end of electrode 309 (on the left-hand sideof FIG. 2A). The end of electrode 309 and the portion of extension 311Bopposite thereto define an opening 313 between the photosensitive areaand the storage area. Well 303 of the photosensitive area extendsthrough opening 313 all the way to well 307 of the storage area, well303 extending from electrode 309 to electrode 311 at the level ofopening 313.

In this embodiment, portions 311A and 311B of electrode 311 areorthogonal and electrode 311 has, in top view, an L shape. Extension311B extends beyond opening 313 and partially delimits an edge of thephotosensitive area adjacent to the edge of the photosensitive areaalong which the storage area extends. The storage area has the samelength as electrode 309 and has short sides aligned with the ends ofelectrode 309. In an alternative embodiment, the storage area is shorterthan electrode 309, one and/or the other of the short sides of thestorage area then being recessed with respect to one and/or the other ofthe ends of electrode 309.

The read area of pixel 300 comprises a heavily-doped N-type region 315(N⁺), more heavily doped than wells 303 and 307. Region 315 extends insubstrate 301 from the upper surface thereof, down to a depth smallerthan that of well 307 of the storage area. Region 315 is arranged on theside of the second end of electrode 309 (on the right-hand side of FIG.2A), opposite well 307 in the extension of the storage area. A portion317 of the substrate separates well 307 from region 315. An insulatedhorizontal gate, or control gate, 318 is arranged on top of and incontact with portion 317 of the substrate. Gate 318 extends from region315 to the storage area and forms the gate of a MOS transistor havingits channel-forming region corresponding to portion 317 of the substrateand having its source and drain regions corresponding to well 307 and toregion 315. In this example, gate 318 partially covers the storage areaand, under gate 318, well 307 of the storage area extends down to theupper surface of substrate 301. Although this is not shown, a drainextension region may extend from region 315 under a portion of gate 318.

An insulated vertical electrode 319 delimits most of the sides of thephotosensitive area which are not bordered with the storage area. Inthis example, electrode 319 has the shape of a U having its horizontalbar delimiting the side of the photosensitive area opposite to the sidebordered with the storage area. Electrode 319 extends in substrate 301down to a depth greater than or equal to that of well 307 of the storagearea. In this example, electrode 319 extends down to the same depth aselectrodes 309 and 311.

An insulated vertical electrode, or counter-electrode, 321 extends inthe substrate parallel to electrode 309. Electrode 321 extends from anedge of the photosensitive area, beyond the second end of electrode 309(on the right-hand side of FIG. 2A), to stop before the first end ofelectrode 309 (on the left-hand side of FIG. 2A). Electrode 321partially delimits the photosensitive area on the side of the storagearea. Electrode 321 extends in the substrate down to a depth greaterthan or equal to that of well 307 of the storage area, for example, downto the same depth as electrodes 309, 311, and 319. As shown in FIG. 2A,electrode 321 may have an L shape, with its long bar parallel toelectrode 309 and its short bar partially delimiting the edge of thephotosensitive area having the long bar extending therefrom. In thisembodiment, electrode 321 is separated from electrode 319. In analternative embodiment, the two electrodes may be joined and correspondto two portions of a same electrode.

To form electrodes 309, 311, 319, and 321, one may, for example, formtrenches vertically extending in substrate 301 from the front surfacethereof, according to a pattern corresponding to the desired electrodeshape. The lateral walls and the bottom of the trenches may be coatedwith an insulating material, for example comprising silicon oxide, afterwhich the trenches are filled with a conductive material. As an example,the conductive filling material may be heavily-doped polysilicon or ametal selected from the group comprising copper and tungsten.

Metallizations (not shown) electrically connect the upper surfaces ofelectrodes 309 and 311 to a node CTRL1, the upper surfaces of electrodes319 and 321 to a node V_(Pol), gate 318 to a node CTRL2, and the uppersurface of region 315 to a node SN′. As an example, the read circuit ofpixel 300 is the same as that of pixel 200, the read circuit then beingconnected to node SN′ of pixel 300 in the same way as to node SN ofpixel 200. The control voltages applied to nodes CTRL1 and CTRL2 of eachsensor pixel are, for example, provided by a sensor control circuit.

Pixel 300 of FIGS. 2A to 2E is intended to receive an illumination onthe upper surface or front surface side of substrate 301. Although thisis not shown, pixel 300 comprises an opaque screen, for example,metallic, located on its upper surface side masking the entire surfaceof the storage area. As an example, the opaque screen masks the entiresurface of the pixel except for the photosensitive area thereof.

An example of an operating mode of pixel 300 will now be described.

In integration phase, nodes V_(Pol) and CTRL1 are at a same referencevoltage. This voltage may be the ground voltage, or may be negative withrespect to ground, for example, in the order of −1 V. Such a biasing ofelectrodes 309, 311, 319, and 321 causes an accumulation of holes alongthe walls of these electrodes. Holes also accumulate along the walls ofopening 313. The depletion voltage of well 303 at the level of opening313 is lower, for example, close to 0 V, than the depletion voltage ofwells 303 and 307, which blocks electron exchanges between wells 303 and307. Substrate 301 is also biased to a reference voltage, for example,the ground voltage. The dimensions of opening 313, the dimensions ofwells 303 and 307, and the doping levels of regions 305, 303, 307, 313,and 301 are selected so that, in the absence of illumination and afterthe charges have been transferred, wells 303 and 307 are fully depleted,in particular at the level of opening 313. As a result, a potential wellforms in well 303 and a potential well forms in well 307, which dependon the doping levels and on the bias voltages of the electrodes and ofthe substrate. When photodiode PD is illuminated, electron/hole pairsare photo-generated in the photodiode, and the photogenerated electronsare attracted towards well 303 and trapped therein.

In a phase of transfer of the photogenerated electrons accumulated inwell 303 of photodiode PD to well 307 of the storage area, node CTRL1 isset to a sufficiently high voltage, for example, between 2 and 4 V, toset the depletion voltage of well 303 at the level of opening 313 to avoltage greater than the maximum potential of the potential well inphotodiode PD during the integration phase. This causes the transfer ofall the photogenerated electrons contained in well 303 to well 307, viaopening 313. During the transfer phase, node V_(Pol) remains at the samereference voltage as during the integration phase. Gate 318 (node CTRL2)is set to a voltage such that the corresponding MOS transistor is in anoff state.

Advantageously, the transfer is eased when extension 311B of electrode311 partially delimits one of the edges of the photosensitive area, asshown in FIG. 2A. Indeed, the photogenerated electrons accumulated inthe photosensitive area are then attracted towards extension 311B whichguides them towards the storage area.

An advantage of pixel 300 of FIGS. 2A to 2E over pixel 200 of FIGS. 1Ato 1F is that the presence of counter-electrode 321 biased to thereference voltage during the transfer avoids for the photogeneratedelectrons present in the photosensitive area to reach the walls ofelectrode 309 and to recombine with holes trapped in interface defects.This enables to eliminate the risk of loss of charge during thetransfer, conversely to the case of pixel 200 of FIGS. 1A to 1F, where,during the transfer, the photogenerated electrons present in thephotosensitive area are attracted towards electrode 203.

Once the transfer has been performed, during a storage phase, node CTRL1is set back to the same low voltage as node V_(Pol) to maintain thetransferred electrons confined in the potential well of well 307, beforea subsequent transfer to read area 315. At this stage, photodiode PDcomprises no photogenerated charge (that is, it is in a reset state),and a new integration phase may start.

In read phase, the electrons contained in the storage area aretransferred to read area 315. To achieve this, gate 318 (node CTRL2) isset to a voltage, for example, between 2 and 4 V, such that thecorresponding MOS transistor is in a conductive state. During thetransfer, nodes V_(Pol) and CTRL1 are maintained at the same referencevoltage of low value as during the integration phase. Once the transferhas been performed, gate 318 is set back to the voltage blocking thecorresponding MOS transistor. At this stage, diode SD comprises nophotogenerated charge (that is, it is in a reset state). To favor thecharge transfer from the storage area to the read area, in analternative embodiment, it is provided for well 307 to be more heavilyN-type doped on the side of its upper surface than on the side of itslower surface.

One of the advantages of pixel 300 over pixel 200 will now be describedin relation with FIGS. 3A and 3B.

In FIG. 3A, the shape of voltage V in the storage area of pixel 200 isillustrated by a curve 401 during an accumulation phase, and by a curve403 at the beginning of a transfer phase. More specifically, each ofthese curves illustrates the shape of the voltage along an axis includedin cross-section plane G-G of FIG. 1A, between a position x₁corresponding to the interface between electrode 203 and well 207, and aposition x₂ corresponding to the interface between electrode 209 andwell 207.

Curve 401 is then obtained while the storage area comprises no chargesand electrodes 203 and 209 are biased to −1 V. As previously indicated,a potential well then forms in the storage area having a maximum valuein a central portion of the storage area (position x₃). Curve 403 isobtained while the storage area still comprises no charges, electrode203 is biased to 2.6 V and electrode 209 remains biased to −1 V. Theincrease of the bias voltage of electrode 203 causes an increase ΔV1 ofthe voltage at position x₁, and an increase ΔV2 of the voltage atposition x₃. However, since electrode 209 remains biased to −1 V, thevoltage at position x₂ remains the same as during the accumulationphase. As a result, voltage increase ΔV2 is smaller than voltageincrease ΔV1. To obtain an increase ΔV1 of the voltage at position x₃,electrode 203 should be biased to more than 2.6 V.

In FIG. 3B, the shape of voltage V in the storage area of pixel 300 isillustrated by a curve 405 during an accumulation phase, and by a curve407 at the beginning of a transfer phase. More specifically, each ofthese curves illustrates the shape of the voltage along an axis includedin cross-section plane D-D of FIG. 2A, between a position x₄corresponding to the interface between electrode 309 and well 307 and aposition x₅ corresponding to the interface between electrode 311 andwell 307. Curve 405 is obtained while the storage area comprises nocharges and electrodes 309 and 311 are biased to −1 V. A potential wellforms in the storage area, and has its maximum value in a centralportion of the storage area (position x₆). Curve 407 is obtained whilethe storage area still comprises no charges and electrodes 309 and 311are biased to 2.6 V. The increase of the bias voltage of electrodes 309and 311 causes an increase ΔV1 of the voltage at positions x₄ and x₅and, conversely to the case of pixel 200, an increase ΔV1 of the voltageat position x₆ can be observed.

Thus, advantageously, to obtain a potential well of a given thicknesssufficient to trap all the charges transferred from the photosensitivearea to well 307 of the storage area, the bias voltage of electrodes 309and 311 is lower than that of electrode 203 of pixel 200. This advantageresults from the fact that, in pixel 300, the storage area is narrow andall the insulated vertical electrodes in contact with the storage areaare electrically interconnected.

Another advantage of pixel 300 of FIGS. 2A to 2E over pixel 200 of FIGS.1A to 1G will now be described.

In FIG. 4A, the shape of voltage V in well 205 of pixel 200 during atransfer phase is illustrated by a curve 411 when electrode 203 isbiased to 2.6 V, and by a curve 413 when the electrode is biased to 4.1V, electrode 202 being biased to −1 V. More particularly, each of thesecurves illustrates the shape of the voltage along an axis included incross-section plane F-F of FIG. 1A, between a position x₇ correspondingto the interface between electrode 202 and well 205, and a position x₈at the level of transfer area 204.

As can be seen in FIG. 4A, when electrode 203 is biased to 2.6 V (curve411), the voltage in well 205 does not monotonously increase all the wayto the transfer area (position x₈). As a result, at the end of atransfer phase, there remain electrons photogenerated during theaccumulation phase in the photosensitive area. An unwanted lagphenomenon can then be observed between two successive pixel readphases. Electrode 203 should be biased to 4.1 V (curve 413) so that thevoltage in well 205 monotonously increases all the way to transfer area204 (position x₈) and that all the photogenerated electrons accumulatedin the photosensitive area are effectively transferred to the storagearea. The use of such a high biasing is not desirable.

In FIG. 4B, the shape of voltage V in well 303 of pixel 300 isillustrated by a curve 415 corresponding to the case of curve 411 ofFIG. 4A, that is, when electrodes 309 and 311 are biased to 2.6 V andelectrode 319 is biased to −1 V. The curve illustrates the shape of thevoltage along an axis parallel to cross-section plane E-E of FIG. 2A,between a position x₉ corresponding to the interface between electrode319 and well 303 and a position x₁₀ at the level of opening 313.

As shown in FIG. 4B, the voltage monotonously increases all the way toopening 313 (position x₁₀) whereby all the electrons are transferred tothe storage area.

Thus, advantageously, the minimum bias voltage of electrodes 309 and 311to suppress remanence phenomena in pixel 300 is lower than that ofelectrode 203 of pixel 200. This advantage results from the fact thatwell 303 of the photosensitive area is thinner than well 205 of thephotosensitive area of pixel 200, and, more particularly, due to thefact that the thickness of well 303 is selected so that, in the absenceof illumination, the value of the potential well in a central portion ofwell 303 (position x₁₁) only depends on the doping levels and isindependent from the biasing conditions of electrodes 309, 311, 319, and321.

As an example, pixel 300 of FIGS. 2A to 2E may have the followingdimensions:

-   -   a length between 1 and 6 μm, for example, 3 μm, and a width        between 1 and 4 μm, for example, 2 μm, for a photosensitive area        which is rectangular in top view and extends lengthwise in the        same direction as the storage area,    -   a length between 1 and 6 μm, for example, 2 μm, and a width        between 0.2 and 1 μm, for example, 0.3 μm, for the storage area,    -   a length between 1 and 6 μm, for example, 2 μm, for portion 311A        of electrode 311,    -   a length between 0.5 and 1.5 μm, for example, 0.8 μm, for        extension 311B of electrode 311,    -   a length between 1 and 6 μm, for example, 1.8 μm, for electrode        309,    -   from 0.1 to 1 μm, for example, 0.2 μm, between the first end of        electrode 309 and extension 311B opposite thereto,    -   from 0.1 to 0.4 μm, for example, 0.275 μm, between the storage        area and region 315 of the read area,    -   from 0 to 100 nm, for example, 50 nm of recess of the storage        area with respect to the first end of electrode 309,    -   from 0 to 100 nm of recess, for example, 70 nm of recess of the        storage area with respect to the second end of electrode 309,    -   a thickness between 0.2 and 1 μm, for example, 0.5 μm for well        303,    -   a thickness between 1 and 10 μm, and preferably between 2 and 4        μm, for well 307,    -   a thickness between 0.1 and 0.5 μm for region 315,    -   a thickness between 100 and 300 nm, for example, 200 nm, for        P⁺-type doped thin layer 305, and    -   a depth between 1 and 10 μm, preferably between 2 and 5 μm, and        a width between 0.1 and 0.5 μm for the insulated vertical        electrodes.

As an example, for a given manufacturing technology, the doping levelsof the various regions of pixel 300 are the following:

-   -   from 10¹⁷ to 10¹⁹ at·cm⁻³, for example, 10¹⁸ at·cm⁻³, for thin        layer 305,    -   from 10¹⁶ to 10¹⁸ at·cm⁻³, for example, 10¹⁷ at·cm⁻³, for well        303,    -   from 10¹⁶ to 10¹⁹ at·cm⁻³, for example, 10¹⁷ at·cm⁻³, for well        307,    -   from 10¹⁹ to 10²² at·cm⁻³, for example, 10²¹ at·cm⁻³, for region        315, and    -   from 10¹⁴ to 10¹⁶ at·cm⁻³, for example, 10¹⁵ at·cm⁻³, for well        301.

FIGS. 5A and 5B schematically show a pixel 300′ corresponding to analternative embodiment of pixel 300 of FIGS. 2A to 2E. FIG. 5A showspixel 300′ in top view and FIG. 5B corresponds to a cross-section viewalong plane B-B of FIG. 5A.

Pixel 300′ comprises same elements as pixel 300 and is similar thereto,with the difference that it further comprises an insulated verticalelectrode 501 arranged between electrode 309 and portion 311A ofelectrode 311. Electrode 501 has substantially the same dimensions aselectrode 319 and extends in well 307 and substrate 301, parallel andopposite to electrode 309. The interval between the opposite surfaces ofelectrodes 309 and 501 and between the opposite surfaces of electrode501 and of portion 311A of electrode 311 is substantially the same asthe interval between the opposite surfaces of electrodes 309 and 311 inpixel 300. Electrode 501 is electrically connected to electrodes 309 and311, that is, to node CTRL1, by metallizations, not shown.

Thus, the storage area of pixel 300′ comprises two rectangular areas, orregions, corresponding to two storage areas of pixel 300 placed side byside and simultaneously controlled by signal CTRL1. This enables todouble the storage capacity of the storage area of pixel 300′ withrespect to that of pixel 300. Further, each of the two rectangular areasof the storage area of pixel 300′ having the same dimensions as thestorage area of pixel 300, the advantage of the storage area of pixel300 described in relation with FIGS. 3A and 3B remains valid for each ofthe rectangular areas of the storage area of pixel 300′.

In alternative embodiments of pixel 300′, it may be provided for thestorage area to comprise more than two rectangular areas by arrangingother electrodes 501 between electrodes 309 and 311.

Specific embodiments have been described. Various alterations,modifications, and improvements will occur to those skilled in the art.The shape of electrodes 309, 311, 319, 321, and 501 in top view (FIGS.2A and 5A) are indicative and may be adapted by those skilled in the artto improve charge transfers and or to decrease the pixel surface area.In particular, although a rectangular storage area has been described,said area may have any desirable elongated shape, for example, atrapezoidal shape. In this last case, the storage area is wider at thelevel of its second end, on the side of the read area, than at the levelof its first end, on the side of the photosensitive area, to favor thecharge transfer from the transfer area to the read area. As an example,for a storage area having a length in the range from 1 to 6 μm, thestorage area may be wider by from 0.1 to 0.5 μm on the side of itssecond end than on the side of its first end. In the alternativeembodiment of FIGS. 5A and 5B, it may also be provided for the storagearea to comprise at least two trapezoidal regions.

Further, the described embodiments may be adapted to other structures ofa pixel with insulated vertical electrodes. In particular, it will bewithin the abilities of those skilled in the art to adapt the describedembodiments to add thereto an anti-dazzle system, for example, such asthat described in U.S. Pat. No. 9,236,407, enabling to avoid, in case ofa saturation of the photosensitive area during an accumulation phase,for an excess of photogenerated charges to pour into the storage area.It will also be within the abilities of those skilled in the art toadapt the described embodiments to sensors where a plurality of pixelsshare a same read area and/or a same read circuit. Further, well 303 ofthe photosensitive area may have substantially the same depth as well307 of the storage area.

Embodiments where counter-electrode 321 is of same nature as electrode319 have been described. Counter-electrode 321 may also be replaced witha wall of an insulating material coated with a heavily-doped P-typelayer (P⁺) or with a heavily-doped P-type semiconductor wall (P⁺).Electrode 321 may also be replaced with an insulating wall coated with aheavily-doped P-type layer.

It will be within the abilities of those skilled in the art to adapt thedescribed embodiments to pixel structures where all the conductivitytypes are inverted with respect to the above-mentioned examples.

The described embodiments are not limited to the example of read circuitshown in FIG. 1A and it will be within the abilities of those skilled inthe art to obtain the desired operation by using other read circuits.

Although pixels intended to receive an illumination on the front surfaceside of the substrate have been described, it will be within theabilities of those skilled in the art to adapt the described embodimentsto the case of pixels intended to receive an illumination from the rearsurface of the substrate. As an example, in the case of an illuminationfrom the rear surface of the substrate, the opaque screen covering thestorage area will be arranged on the side of this rear surface, and,further, the substrate may be thinned from its rear surface all the wayto the insulated vertical electrodes.

Such alterations, modifications, and improvements are intended to bepart of this disclosure, and are intended to be within the spirit andthe scope of the present invention. Accordingly, the foregoingdescription is by way of example only and is not intended to belimiting. The present invention is limited only as defined in thefollowing claims and the equivalents thereto.

The invention claimed is:
 1. An image sensor including a plurality ofpixels, each pixel comprising: a semiconductor substrate doped with afirst dopant type; a first insulated vertical electrode that delimits aphotosensitive area doped with a second dopant type, wherein the firstinsulated vertical electrode is electrically connected to receive afirst bias voltage; a second insulated vertical electrode and a thirdinsulated vertical electrode that delimit a charge storage area dopedwith said second dopant type, wherein the second and third insulatedvertical electrodes are physically separate from each other butelectrically connected to receive a second bias voltage; wherein saidfirst insulated vertical electrode and said third insulated verticalelectrode extend parallel to each other with a first portion of saidsemiconductor substrate doped with the first dopant type positioned incontact with and extending between said first and third insulatedvertical electrodes; wherein the third insulated vertical electrodeextends between the charge storage area and the photosensitive area; andwherein the second insulated vertical electrode extends perpendicular tothe third insulated vertical electrode at a first end of the thirdinsulated vertical electrode to delimit a region for charge passagebetween the photosensitive area and the charge storage area in responseto the second bias voltage.
 2. The image sensor of claim 1, furthercomprising a read area separated from the charge storage area by asecond portion of said semiconductor substrate doped with the firstdopant type.
 3. The image sensor of claim 2, further comprising aninsulated control gate arranged on top of said second portion of thesemiconductor substrate, the insulated control gate extending from theread area to the charge storage area.
 4. The image sensor of claim 3,wherein the insulated control gate is configured to receive a controlvoltage configured to control transfer of charge from the charge storagearea to the read area.
 5. The image sensor of claim 1, wherein thecharge storage area has an elongated rectangular shape in plan viewhaving a width and a length, said length being at least five timeslonger than said width.
 6. The image sensor of claim 1, wherein thesecond bias voltage has a first voltage level configured to permit acharge transfer from the photosensitive area to the charge storage area,and has a second voltage level which blocks charge transfer from thephotosensitive area to the charge storage area.
 7. The image sensor ofclaim 1, wherein the photosensitive area and the charge storage area arecovered with a layer doped with the first dopant type.
 8. The imagesensor of claim 1, wherein the first bias voltage is a constant biasvoltage and wherein the second bias voltage is a variable bias voltage.9. An image sensor including a plurality of pixels, each pixelcomprising: a semiconductor substrate doped with a first dopant type; afirst insulated vertical electrode that delimits a photosensitive areadoped with a second dopant type, wherein the first insulated verticalelectrode is electrically connected to receive a first bias voltage; asecond insulated vertical electrode and a third insulated verticalelectrode that delimit a charge storage area doped with said seconddopant type, wherein the second and third insulated vertical electrodesare physically separate from each other but electrically connected toreceive a second bias voltage; wherein said first insulated verticalelectrode and said third insulated vertical electrode extend parallel toeach other with a first portion of said semiconductor substrate dopedwith the first dopant type positioned in contact with and extendingbetween said first and third insulated vertical electrodes; and whereinthe third insulated vertical electrode extends between the chargestorage area and the photosensitive area, and wherein the secondinsulated vertical electrode comprises a first portion extendingparallel to a length of the third insulated vertical electrode and asecond portion extending perpendicular to the first portion opposite afirst end of the third insulated vertical electrode.
 10. The imagesensor of claim 9, further comprising a read area separated from thecharge storage area by a second portion of said semiconductor substratedoped with the first dopant type.
 11. The image sensor of claim 10,further comprising an insulated control gate arranged on top of saidsecond portion of the semiconductor substrate, the insulated controlgate extending from the read area to the charge storage area.
 12. Theimage sensor of claim 11, wherein the insulated control gate isconfigured to receive a control voltage configured to control transferof charge from the charge storage area to the read area.
 13. The imagesensor of claim 9, wherein the charge storage area has an elongatedrectangular shape in plan view having a width and a length, said lengthbeing at least five times longer than said width.
 14. The image sensorof claim 9, wherein a first end of the charge storage area emerges ontothe photosensitive area at said second portion.
 15. The image sensor ofclaim 14, further comprising a read area separated from a second end ofthe charge storage area by a second portion of said semiconductorsubstrate doped with the first dopant type.
 16. The image sensor ofclaim 9, wherein the second bias voltage has a first voltage levelconfigured to permit a charge transfer from the photosensitive area tothe charge storage area, and has a second voltage level which blockscharge transfer from the photosensitive area to the charge storage area.17. The image sensor of claim 9, wherein the photosensitive area and thecharge storage area are covered with a layer doped with the first dopanttype.
 18. An image sensor including a plurality of pixels, each pixelcomprising: a semiconductor substrate doped with a first dopant type; afirst insulated vertical electrode that delimits a photosensitive areadoped with a second dopant type, wherein the first insulated verticalelectrode is electrically connected to receive a first bias voltage; asecond insulated vertical electrode and a third insulated verticalelectrode that delimit a charge storage area doped with said seconddopant type, wherein the second and third insulated vertical electrodesare physically separate from each other but electrically connected toreceive a second bias voltage; wherein said first insulated verticalelectrode and said third insulated vertical electrode extend parallel toeach other with a first portion of said semiconductor substrate dopedwith the first dopant type positioned in contact with and extendingbetween said second and third insulated vertical electrodes; and afourth insulated vertical electrode positioned between the second andthird insulated vertical electrodes that divides the charge storage areainto a first charge storage area and a second charge storage area,wherein the second, third, and fourth insulated vertical electrodes arephysically separate from each other but electrically connected toreceive said second bias voltage.
 19. The image sensor of claim 18,further comprising a read area separated from the first charge storagearea by a second portion of said semiconductor substrate doped with thefirst dopant type.
 20. The image sensor of claim 19, further comprisingan insulated control gate arranged on top of said second portion of thesemiconductor substrate, the insulated control gate extending from theread area to one the first and second charge storage areas.
 21. Theimage sensor of claim 20, wherein the insulated control gate isconfigured to receive a control voltage configured to control transferof charge from one the first and second charge storage areas to the readarea.
 22. The image sensor of claim 18, wherein each of the first andsecond charge storage areas has an elongated rectangular shape in planview having a width and a length, said length being at least five timeslonger than said width.
 23. The image sensor of claim 18, wherein thesecond bias voltage has a first voltage level configured to permit acharge transfer from the photosensitive area to one of the first andsecond charge storage areas, and has a second voltage level which blockscharge transfer from the photosensitive area to one of the first andsecond charge storage.
 24. The image sensor of claim 18, wherein thethird insulated vertical electrode extends between one of the first andsecond charge storage areas and the photosensitive area, and wherein thesecond insulated vertical electrode comprises a first portion extendingparallel to a length of the third and fourth insulated verticalelectrodes and a second portion extending perpendicular to the firstportion opposite a first end of each of the third and fourth insulatedvertical electrodes.
 25. The image sensor of claim 24, wherein a firstend of each of the first and second charge storage areas emerges ontothe photosensitive area at said second portion.
 26. The image sensor ofclaim 25, further comprising a read area separated from a second end ofeach of the first and second charge storage areas by a second portion ofsaid semiconductor substrate doped with the first dopant type.
 27. Theimage sensor of claim 18, wherein each of the first and second chargestorage areas has an elongated rectangular shape in plan view having awidth and a length, said length being at least five times longer thansaid width.
 28. The image sensor of claim 18, wherein the photosensitivearea and the first and second charge storage areas are covered with alayer doped with the first dopant type.